1. Field of the Invention
The present invention relates to a length measuring apparatus having a scale extended in a length measuring direction and a slider including a sensor moving in the length measuring direction along the scale relative thereto in a state of maintaining a predetermined interval between the scale and the slider. Particularly, the present invention relates to a length measuring apparatus preferably used in a linear scale or a length measuring unit and promoting reliability by avoiding an obstacle in mechanism against invasion of a foreign matter from outside.
2. Description of the Related Art
In a measuring apparatus of a linear scale or the like, there are guide mechanisms for ensuring a clearance between a scale and a detecting sensor by rolling (bearing) and sliding.
An example of a linear scale adopting the latter is shown by FIG. 6 (sectional view), FIG. 7 (right side view of a slider portion) and FIG. 8 (left side view of the same). The linear scale is provided with a scale 12, a slider 20, and a slider shoe 22. The scale 12 is extended in a length measuring direction (a direction orthogonal to paper face of FIG. 6). The slider 20 is moved in the length measuring direction along the scale 12 relative thereto in a state of maintaining a predetermined interval to the scale 12. The slider 20 includes a light source, an index scale and a sensor of a light receiving element (case of photosensitive type), an electrode element (case of electrostatic capacitor type), a receiving coil (case of electromagnetic induction type) or the like. The slide shoe 22 is fixed to the slider 20 for maintaining the interval between the scale 12 and the slider 20.
The scale 12 is fixed to a scale frame 10 fixed to one member of relative movement. A detecting unit 24 is fixed to other member of relative movement and a connecting rod 26 transmits relative movement displacement of the detector unit 24 to the slider 20. The slide shoe 22 slides on guide faces 11 provided, for example, on upper and lower sides of the scale 12 of the scale frame 10.
When the sliding mechanism is adopted, as shown by FIG. 9, when a foreign matter 30 is accumulated on front and rear sides of the slider shoe 22 and the foreign matter 30 is brought into an interval between the slide shoe 22 and the sliding guide face 11, there poses a problem that the clearance cannot be maintained and erroneous operation is produced.
In order to resolve such a problem, JP-A-9-189574 proposes to arrange an absorbing member capable of absorbing a liquid as a cleaning member on front and rear sides of a slider moving along a scale. Further, JP-A-2001-255134 discloses to provide wipers on front and rear sides of a slider to sweep to clean a surface of a scale.
Further, it has been conceived to alleviate an influence of a foreign matter to a sensor portion by changing a type of detection of a length measuring apparatus from a photoelectric type to an electromagnetic induction type.
However, according to the example of the related art shown in FIG. 6 through FIG. 8, the slide shoe 22 is constituted by a rectangular parallelepiped and therefore, as shown by FIG. 9, there poses a problem that in accordance with movement of the slider 20, the foreign matter 30 is accumulated on the front and rear sides and the foreign matter 30 cannot be excluded.
Further, according to methods disclosed in JP-A-9-189574 and JP-A-2001-255134, the absorbing element or the wiper is brought into contact with a face of the scale and therefore, there is a concern of bringing about a measuring error by producing a scratch.
Further, even when a sensor portion per se is provided with a resistance against a foreign matter as in the electromagnetic induction type, an obstacle of mechanism of wear, destruction of pattern or the like produced by bringing the foreign matter into the interval between the scale and the sensor moved relative to each other cannot be avoided.